Regulation Of Thermal Stability Structures During Preoxidization For Polyacrylonitrile Fibers

A series physical changes and chemical reactions occur in polyacrylonitrile(PAN)fibers during preoxidization process,leading to the formation of aromatic trapezoidal structure that can sustain the subsequent heattreament in carbonization.The formation of oxidized structures and cyclic structures can enhance the thermal stability of PAN fibers,that is to say,it can reduce the mass loss caused by thermal cracking.But the higher content of oxidized structures and cyclic structures does not always result the better thermal stability and performance of PAN fibers because of the complex influence mechanism.Besides,the ratio of oxidized structures and cyclic structures also affect the stability of PAN fibers.It is necessary to control the preoxidization process reasonably to regulate the structures of preoxidized fibers.PAN fibers were heated under different temperature,atmosphere and time to prepare preoxidized fibers with different structures.Using fourier transform infrared spectroscopy(FT-IR),X-Ray electron energy spectrum(XPS),13C-solid nuclear magnetic resonance(13C-NMR),thermal gravity(TG)and other characterization methods,generation of thermal stability structures of preoxidized fibers,impact of temperature,time and atmosphere on preoxidization,and influence mechanism of the oxidized structures and cyclic structures on thermal stability were investigated.The effect of preoxidized structures on the structures and properties of the resulted CF were demonstrated.Results showed as follows:1.The evolution of oxygen-bearing structures can't enhance thermal stability of PAN fibers,besides,the occur of crosslink reaction is the precondition for improvement of thermal stability.The crosslink structure is mainly composed of C=O,O-C=O,C=C,C=N and their conjugated structure C=N-C=N.When the preoxidization degree is below 0.18(ratio of oxygen content to carbon content),the higher content of oxidized structures and cyclic structures,the better the thermal stability of preoxidized fibers.2.When reaction degree below a certain value,cyclization,dehydrogenation,oxidation degree rise fast with the ramp of temperature or time prolongation.However,above the reaction degree,cyclization and dehydrogenation basically remain unchanged,while,oxidation degree keep rising at high speed.Oxidized structures have different transformation pattern under different atmosphere.Temperature rising is benefit for the formation of carboxyl or ester group under air,while,it is tend to form more carbonyl group under nitrogen.So the purpose of regulating relative content of cyclic structures and oxidized structures through controlling temperature and time of preoxidization can be achieved.3.The increase of cyclic structures content enhances thermal stability of preoxidized fibers and is benefit for improving order degree of carbon fibers.While,too much oxidized structures decrease PAN thermal stability by removing carbon atoms during carbonization due to more sp3 hybrid carbon atoms produced.This decreases order degree of carbon fibers and has opposite effect on its performance.On the premise of fully cyclization,improving the ratio of cyclic structure against oxidized structure is benefit for forming low temperature carbon fibers with more perfect structure.4.On the basis of sufficient cyclization,improving the ratio of the relative content ratio of cyclic structures against oxidized structures promotes the growth of carbon fiber graphite crystallite along La direction and the stack thickness of graphite layer,but does not affect the graphite layer spacing.Due to the difference of the oxidization speed and cyclization speed,a post treatment process under inert atmosphere at high preoxidization temperature is proposed to achieve preoxidized fibers with more thermal stable structures.As a result,the carbon yield and mechanical performance of carbon fibers are improved.The formation,evolution and regulation of thermal stability structures are illustrated in this thesis.By preparing preoxidized fibers with optimized structures via post treatment in inert atmosphere,carbon fibers with high carbon yield,mechanical performance and volume density are obtained,which confirms the influence of oxidized structures and cyclic structures on structures and performance of carbon fibers.